North Korea conducted its third nuclear weapons test this evening, the country's official news agency said, and apparently the blast had a higher explosive yield than the earlier tests.

The 5.1-magnitude artificial tremor (initially measured at magnitude 4.9) struck before noon Tuesday local time in North Hamgyong Province. Initially, South Korean Defense Ministry spokesman Kim Min Seok told Bloomberg it was "likely" a nuclear test, adding that more investigation was necessary. Meanwhile, a U.N. Security Council diplomat said the seismic activity was the result of a nuclear test.

The Korean Central News Agency later confirmed the suspicions. "The test was carried out as part of practical measures of counteraction to defend the country's security and sovereignty in the face of the ferocious hostile act of the U.S.," CNN quoted the North Korean agency as saying in a statement that referred to U.S. activity in imposing sanctions after earlier North Korean long-range missile testing.

The size of the quake suggests the explosion was "far larger" than the earlier two nuclear tests, The New York Times reported, though still less powerful than the U.S. bomb set off over Hiroshima.

The U.S. Geological Survey estimated that the disturbance occurred at a depth of about 1 kilometer. A 1-kiloton nuclear test in October 2006 generated a 3.9-magnitude disturbance, while a 2-kiloton test in May 2009 created a 4.5 quake.

The Comprehensive Nuclear-Test Ban Treaty Organization, the international organization tasked with enforcing a treaty that outlawed nuclear tests, put out this statement:

Today our monitoring stations picked up evidence of an unusual seismic event in the Democratic People's Republic of Korea (DPRK). The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK nuclear tests. For now, further data and analysis are necessary to establish what kind of event this is. If confirmed as a nuclear test, this act would constitute a clear threat to international peace and security, and challenges efforts made to strengthen global nuclear disarmament and non-proliferation, in particular by ending nuclear testing.

Updated at 12:58 a.m. PT February 12with confirmation from North Korean news agency and further details.

Updated at 5:15 a.m. PT February 12to correct the day of the latest nuclear test.

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Stephen Shankland has been a reporter at CNET since 1998 and covers browsers, Web development, digital photography and new technology. In the past he has been CNET's beat reporter for Google, Yahoo, Linux, open-source software, servers and supercomputers. He has a soft spot in his heart for standards groups and I/O interfaces.
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